1. Field of the Invention
The invention relates to a valve apparatus for use in a subterranean fluid transmission conduit or work string in which fluid pressure above the valving apparatus can be used to open the valve to permit fluid pressure to act at a location below the valve.
2. Description of the Prior Art
There are many downhole tools and operations which require the injection of fluid through a tubing conduit to the producing formation or some other location within the well or which require the use of fluid pressure applied through the tubing. For example, washing or acidizing operations require fluid injection through the tubing bore. Some tools also incorporate fluid pressure actuated expandable packing elements which are expanded into sealing engagement with the wall of the casing upon the application of fluid pressure within the tubing or the tool bore.
Such tubing pressure dependent operations or well tools generally not only require the injection of fluids through the tubing bore or the application of pressure within the tubing, but often require control over the relative fluid pressures in both the tubing and in the annulus between the tubing or fluid transmission conduit and the well bore or casing. For example, the pressure actuation of the downhole tool, such as a tool having expandable packing elements, may be dependent upon the force generated by the pressure differential existing between the tubing and the annulus. To set or expand the packing elements, the pressure in the tubing, in general, must exceed the pressure in the annulus. Conversely to permit the expanded packing elements to relax, the pressure in the tubing must generally be less than the pressure in the annulus. In low fluid level wells, the annulus fluid pressure may be continuously less than the hydrostatic pressure in the fluid transmission conduit or work string. Thus any operation dependent upon a greater pressure in the annulus than in the tubing would be difficult to perform. For example, a well tool having an expandable packing element actuated by excess fluid pressure in the tubing may be difficult to retract when tubing pressure is reduced because the hydrostatic pressure in the tubing will still exceed the pressure acting on the packing elements in the annulus. Similarly, the excess pressure differential in the tubing may prevent the movement of a tool having a cup type packing element because of the pressure difference between the tubing and the annulus. A simple reliable apparatus for controlling the pressure in the tubing and in the annulus of certain wells, such as wells having a low fluid level, and of equalizing the pressure between the tubing and the annulus is therefore highly desirable. In addition to controlling the relative pressures in the tubing and the annulus, it is highly desirable that such a tool also controls the fluid levels in the tubing and in the annulus during pressure changes. For example, when the pressure is reduced in the tubing, sudden fluid level surges within the tubing, possibly resulting in surface contamination, should be avoided. It is therefore an object of this invention to provide such a tool especially adapted for use in low fluid level wells.